Sleep plays a significant role in cold adaptation. It should be noted that cold adaptation in nature is formed by periodic contact with cold factor and takes place on the background of circadian changes in physiological state of the organism. Now two main experimental methods of cold adaptation are widely used: long-term (LTA) and short-term acclimation (STA), which influence on sleep structure is studied insufficiently. Thus, the aim of our work was the study of changes in sleep structure of rats after LTA and STA. Experiments were performed in Wistar rats (7-8 months’, 220-250 g body weight) and were approved by the Committee of Bioethics at IPC&C NAS of Ukraine. The animals were individually kept in the cages in sound-attenuated chamber with 12:12-h light:dark cycle, Ta=22-24°C, with water and food ad libitum. STA was carried out by exposing the rats to a temperature of -12 or +10°C for 15 min every hour during the day, for a total nine exposures. This procedure was repeated on the second day. LTA was produced by keeping rats at +4°C for 4 weeks with free food and water access. Brain bioelectrical activity 2 days prior and 2 day after cold acclimations was manually stage scored as rapid eye movement (REM) sleep, slow wave sleep (SWS) or wake in 4-s epochs. Statistical data processing was performed using ANOVA. LTA led to the significant SWS amount increase (from 54.8 ± 2.7 to 72.5 ± 4.6%) and REM sleep increase (from 7 ± 1.1 to 10.6 ± 0.6%) by reducing the wakefulness amount (from 38.2 ± 3 to 16.9 ± 6%). It should be noted that sleep in rats, like nocturnal animals, is characterized by greater amount in the light period of the day (60-70%). LTA led to equalization of diurnal distribution of sleep stages (for SWS - 72.9 ± 7.1% in light period and 72.2 ± 12.1% - in dark period, for REM sleep 10.5 ± 0.6% and 10.6 ± 0.5%, respectively). STA (-12°C) led to the REM sleep increase (from 6.2 ± 1.2 to 12 ± 1.5%) in the light period only due to increasing the duration of its episodes (from 95.6 ± 9.3 to 154 ± 9.2s) against decreasing of wakefulness amount (from 47.4 ± 11.1 to 26.7 ± 3.6%). SWS amount did not change, but there was increase in the duration of its episodes (from 152 ± 15.6s to 308 ± 9.2s) against the background of reduction of its number (from 51.3 ± 13 to 21.5 ± 2) during light period of the day. No changes in daily SWS amount were found after STA (+10°C). There was only the tendency to reduce SWS amount in the first 3h after STA in the first day of cold exposures. And SWS increase after ending of STA in both first and second day of exposures was observed. Thus, both types of acclimation change sleep structure and cyclicity in different manner and it seems possible that the mechanisms involved are fundamentally different and depend on character of cold influence (rhythmic character) and degree of temperature load (cold exposure duration).